Iran has already blamed Israel for the assassination of Professor Mostafa Ahmadi Roshan. A website identified with President Mahmoud Ahmadinejad said on Thursday that Iran is preparing its own answers to the string of assassinations of Iranian scientists, adding those responsible had better start staying awake at night.

The website specifically blamed Israel for the killings, saying that Israel wasn’t the only country that could undertake long-distance proxy wars. “We too can send our agents to kill people in their sleep,” the site quoted an unnamed official as saying.

While Panetta did not specify who he believed was behind the killing, earlier on Thursday U.S. President Barack Obama called Israeli Prime Minister Binyamin Netanyahu on the phone and reportedly wanted explanations for the assassination.

Israel so far has not commented on the incident, but IDF spokesman Brigadier General Yoav Mordechai wrote on his Facebook page Wednesday, “I do not know who brought the Iranian scientist to account, but I certainly won’t shed a tear for him.”